The present invention relates to limited scan antennas, and more particularly to a high efficiency, relatively low cost antenna for scanning a narrow beam over a specified angular section with maximum possible gain consistent with the aperture size while using the minimum number of active elements.
The conventional phased array with one phase shifter per element scans a narrow beam many beamwidths within a sector of perhaps .+-.60.degree. from broadside. The angular coverage of such a wide angle scan antenna is illustrated in FIG. 1. A limited scan antenna scans a narrow beam only a few beamwidths about some nominal position, often broadside. The angular coverage of such a limited scan antenna is depicted in FIG. 2. Limited scan systems find use in several applications including:
(i) Weapon locator radars; PA1 (ii) Microwave landing systems; PA1 (iii) Satellite communication systems; and PA1 (iv) Adaptive antennas.
In the first application, accurate trajectory measurements are required early in the flight of a projectile in order to ascertain the source. Narrow high gain beams are required to combat noise and minimize multipath effects, but only a few beamwidths of scan are necessary. The same considerations apply to blind landing systems. The third application requires a narrow high gain beam emanating from a satellite and covering only a portion of the earth--perhaps half a continent. The total number of such beams required to cover the earth is moderately small and the viewing angle of the earth from a satellite in geosynchronous orbit is only 18.degree.. Communication may be accomplished with immunity from interference arising outside a single beam coverage.
A more recent application of limited scan antennas is for use in adaptive arrays. The active modules in such an antenna may be phase shifters and attenuators which are set by control circuitry designed to minimize interference at the output in the receive mode. The terminals attached to the active elements each produce subarray distribution in the aperture. The subarray distributions are virtually identical for each terminal. The corresponding patterns provide the highest possible gain and largest grating lobe suppression possible within the desired limited field of view. This provides greater signal-to-noise and virtually no spurious grating lobe responses. In addition, since the subarrays are all alike, very fast adaptive algorithms such as the Maximum Entropy Method may be employed.
Limited scan antenna designs attempt to provide the same gain and sidelobe performance as a complete phased array with the same aperture. Because only a few beamwidths of scan are required it seems reasonable to expect that it should not be necessary to provide one phase shifter per aperture element to perform the limited scan function. Since the phase shifters and phase shifter drivers are typically the most expensive items in a phased array and these units also are the principal contributors to availability reliability indices of antenna performance, the objective of a limited scan antenna design is to minimize the number of active components without incurring an inordinate growth in the complexity of the passive equipment or a degradation in gain and sidelobe performance. However, the latest technological trend is to distribute solid state transmit amplifiers, receive preamps, phase shifters, and like active devices through the array.
Limited scan capability can be provided using constrained circuitry, i.e., circuitry wherein the rf energy is confined by transmission lines. A standard for comparison is a system comprising a large Butler matrix fed by a small Butler matrix. Such a system is described, for example, in "A Multiple-beam Antenna Feed Network," C. Rothenberg and S. Milazzo, Radiation Division, perry Gyroscope Co., June, 1965. Butler matrices are well known in the art and are described, for example, in the paper "An Electrically Scanned Beacon Antenna," A. E. Holley, E. C. DuFort and R. A. Dell-Imaguire, IEEE Trans. AP-22, Jan., 1974, page 3. The large Butler matrix is a network which produces simultaneous high gain beams but only a few are used for limited scan. The small Butler matrix, in conjunction with the phase shifters and uniform power divider, slides the terminal weighting of the large Butler matrix to steer the beam. This system is optimal in that the fewest number of active elements (equal to the number of beamwidths of scan) is used, the gain is maximized and the levels of the grating lobes are low. However, it is a totally constrained system which is impractical in many cases where even the small Butler matrix is too large, heavy and expensive.
In a survey article, Mailloux reviewed a hybrid scheme utilizing a bootlace aperture lens and a Butler matrix. R. J. Mailloux, "Phased Array Theory and Technology," Proc. IEEE 70, No. 3, March 1982, page 246 et. seq. Although performance of such a scheme is better than the purely optical approaches available at that time the Butler matrix may be too large for practical application--especially for three dimensional cases.
Researchers have sought the optical equivalent of the Butler/Butler limited scan technique. U.S. Pat. No. 3,835,469, of which the present applicant is a co-inventor, discloses a lens type optical scheme which has low phase error. The illumination of the aperture by the small array and correction lens does not stay fixed as the beam is scanned. There is spillover loss on one side and underillumination on the other. This problem can be corrected only by using more than the minimum number of elements.
A second purely optical approach is described in the report by C. H. Tang and C. F. Winter, "A Study of the Use of a Phased Array to Achieve Pencil Beams Over a Limited Sector Scan," AFCRL TR-7300482, ER-73-4292, Raytheon Company, Final Report Contract F19628072-C-0213, AD 768 618. With this approach, a corrective bootlace lens is placed in the focal region. The feed array is focused to a point on the corrective lens and the focal distribution is mapped onto the aperture side of the lens. This focal distribution in turn illuminates the aperture. The beam is scanned in the far field by moving the focal point along the feed side of the corrective lens using the feed array phase shifters. Although the system is geometrically focused for all scan angles, the aperture illumination slides off to one side as the beam scans, resulting in spillover at one end and under-illumination at the other end of the aperture. The system is very efficient up to half the maximum scan angle if the corrective lens radii are optimized empirically; however, gain is still much lower than the Butler matrix technique at maximum scan. The only apparent remaining ways to improve the approach is to use about twice the theoretical minimum number of elements or use a large under-illuminated aperture.
It would therefore represent an advance in the art to provide an optical limited scan antenna which employs the smallest possible aperture and the minimum number of active elements while maintaining nearly 100% efficiency for all angles within the limited field of view.